One in 80 of all naturally conceived live births are twins, but many more than this are conceived. That's because sometimes, at an early stage, one embryo simply stops growing and is absorbed into your uterus wall.

Ultrasound technology is now so advanced that it can tell us all sorts of things that may have once gone unnoticed. A vaginal scan can locate two gestational sacs when you're just five weeks pregnant.

These sacs surround your developing babies. At this stage the sacs appear empty, but a few days later your sonographer may see heartbeats and yolk sacs. Yolk sacs provide babies with their first nutrients. Tiny embryos can be seen from about six weeks, when they're just 3mm (0.12in) long.

Once a twin pregnancy has been found early on, about one in five will have one of the two sacs disappear before 12 weeks.

It's worth mentioning that one of the two embryos could, sadly, stop growing at any stage. An embryo could grow for eight weeks or more, be seen during the dating scan, but not survive later due to complications.

You've probably spent several weeks between the two scans imagining what it would be like to have twins. It's only natural if you now feel a sense of sadness and loss.

Rest assured that your pregnancy can continue as normal. If you're having a combined screening test for Down syndrome, as long as there's no sign of the vanished twin, it will still be accurate.

In some cases, the second sac with a tiny undeveloped embryo is still visible during the combined screening test. This may influence the blood test results, so the screening result would be calculated on the scan alone.

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